Louise was a young woman from a planet originally almost identical to 1960s Earth. Like all survivors on her world, Louise was infected with the life prolongation complex. When the USS Enterprise landing party encountered her in 2266, she was over three hundred years.
That same year, as she entered puberty, she was completely transformed by her infection. Her fellow children ran from her screaming, then she attacked Captain James T. Kirk. She died after Kirk shot her with a phaser set on stun. She was a little older than her acquaintance, Miri. (TOS: "Miri")